Riot police used teargas to disperse crowds tonight in a series of tense standoffs with anti-globalisation protesters a few miles from the convention centre staging this week's G20 summit.

As the heads of the world's most powerful countries gathered to discuss the global economy, hundreds of mainly youthful demonstrators overturned dustbins to form makeshift barricades in the working class Pittsburgh neighbourhood of Lawrenceville.

Some in the crowd covered their faces with scarves or balaclavas, many were dressed in black and waved black flags. A police presence, including Swat teams and riot squads, from as far afield as Kentucky and Virginia blocked them from marching towards the city centre.

Using powerful megaphones mounted on a truck, the police announced in English and Spanish: "By order of the City of Pittsburgh chief of police, I hereby declare this to be an unlawful gathering. I order all of those present to immediately disperse."

At one stage, the authorities deployed teargas and they repeatedly used an ear-splitting siren to get bystanders to leave. A pattern developed as the crowd dispersed before reconvening a few streets away and marching to taunt lines of stony-faced police.

On the corner of Penn Street and 33rd Street, a line of police slowly advanced, thumping their riot shields with batons. The crowd, backing away, yelled: "USA, USA" and "Whose street? Our street."

Stephen Boatwright, 21, a student from New York, said he objected to the G20 leaders convening behind closed doors. "I don't think Barack Obama should be making macro economic or foreign policy decisions without my input," he said.

Pointing to the multi-billion dollar bailout of Wall Street banks, he added: "Even though Barack Obama won an election, largely corporate and special interests are controlling his policies."

Demonstrators carried placards with slogans such as "Fuck world trade" and "Greedy 20 slave lords". A handful threw stones at police and several were arrested. A second man, in his 60s, was arrested after sitting in front of an advancing riot truck, to a chorus of catcalls from bystanders.

The protesters organised by a group called the Pittsburgh G20 Resistance Project which describes itself as a forum for a variety of causes including environmental concerns, anti-poverty and anti-capitalist demonstrators. A spokesman, Noah Williams, said the overarching theme was "anti-hierarchical and anti-authoritarian."

Some of those protesting had travelled across the US to make their views known. Sue Bastian, a retired woman from Oregon, carried a black umbrella with slogans asserting that G20 leaders were in favour of wars, fascism, corporate rule, and police states. She said: "The G20 is a collection of politicians from the 20 wealthiest, most powerful nations on the face of the planet. They are control freaks."

Inside a tight security cordon in the centre of Pittsburgh, the scene was calmer. As a welcoming committee prepared to greet the Chinese leader Hu Jintao outside the Westin hotel, four trailers were positioned across the road to block a view of a few dozen protesters, some of whom wore T-shirts of the Falun Gong group which is outlawed in China.

Pittsburgh's mayor, Luke Ravenstahl, said that the cost of the summit in public safety terms was $18m (£11.2m) but that he anticipated an economic benefit to the city of $20-30m.

He accepted that some citizens were unhappy about the disruption but said: "We've asked them to understand that the short-term sacrifice they're going to have to make is worth the long-term benefits to the city."